A locking key of this general type has been described in German laid-open specification (Offenlegungsschrift) No. 26 34 969, published Feb. 9, 1978. The key of that publication has a manually grippable head member and a bit-carrying shank member which are limitedly relatively rotatable about an axis parallel to the longitudinal direction of the shank member; the latter has an enlarged tubular neck which accommodates a cylindrical boss of the head member centered on the axis of rotation. In one disclosed mode of realization, the boss is surrounded by a friction ring forming a lost-motion rotary coupling between the head and the shank, the tubular neck having a window enabling the viewing of color markings which visually distinguish between the two limiting relative positions. In another disclosed mode of realization, the friction ring is replaced by two oppositely effective one-way clutches through which the head can entrain the clutch in one or the other direction of rotation upon being shifted into a respective axial position.